1. Field of the Invention
The present invention relates to an amplifier for an acceleration sensor, that is used as being integrated with an air bag device for use in an automobile, and more particularly, to an amplifier for amplifying an output of an acceleration sensor in which a piezoelectric element is used.
2. Description of the Related Art
Conventionally, in an air bag device for use in an automobile, a piezoelectric acceleration sensor is utilized for outputting an actuation signal of the air bag device. That is, by detecting an acceleration that results from a collision and the like of an automobile with an acceleration sensor, the air bag is activated by an output of the acceleration sensor.
In the above-mentioned piezoelectric acceleration sensor, a piezoelectric element is displaced by an acceleration to be applied, and an output voltage that corresponds to a charge generated in the piezoelectric element according to a piezoelectric effect is extracted. In order to amplify this output voltage, conventionally, an amplifier indicated with a dotted line A in FIG. 4 is used.
As shown in FIG. 4, one end of a piezoelectric acceleration sensor 51 is connected to a reference voltage Vr, and the other end thereof is connected to a non-inverting input terminal of a first operational amplifier 52 of a MOS-FET input type. A leakage resistor Ra is connected between the non-inverting input terminal 52 and the reference voltage.
Between the inverting input terminal of the first operational amplifier 52 and the reference voltage, a resistor Rb is connected.
Further, between the output terminal and the inverting input terminal of the first operational amplifier 52, a resistor Rc and a capacitor Ca are connected in parallel to each other. Moreover, the output terminal of the first operational amplifier 52 is connected to a non-inverting input terminal of a second operational amplifier 53 of a MOS-FET input type via a capacitor Cb. A resistor Rd is connected between the reference voltage and the non-inverting input terminal of the second operational amplifier 53. Further, a resistor Re is connected between an output terminal and a inverting input terminal of the second operational amplifier 53. Furthermore, a trimming resistor Rf is connected between the inverting input terminal of the second operational amplifier 53 and the reference voltage.
In the above-mentioned amplifier circuit A, the output terminal of the second operational amplifier 53 is connected to an output terminal 54, and from the output terminal 54, the acceleration sensor 51 outputs an output voltage Vo.
Also, a magnitude of the output voltage Vo is to be adjusted by adjusting a resistance value of the trimming resistor Rf.
In the above-mentioned amplifier circuit A, when a capacitance of the piezoelectric element in the accelerator sensor 51 is Cg, a high pass filter is provided with the capacitance Cg and the leak resistance Ra, and then after having passed through this high pass filter the input voltage Vin is provided to the amplifier A.
A cut-off frequency FL.sub.1 according to the above-mentioned high pass filter is represented by the following equation (1). ##EQU1##
Assuming a gain of the above-mentioned operational amplifier circuit A to be Ag, a cut off frequency of low-frequency band to be FL.sub.a (Hz), and a cut off frequency of high-frequency band to be FH.sub.a (HZ), the gain Ag, the cut off frequency FL.sub.a of low-frequency band and the cut off frequency FH.sub.a of high-frequency band are expressed in the following equations, respectively. ##EQU2##
Further, the above-mentioned Ag, the low-frequency cut off frequency FL.sub.a, and the high-frequency cut off frequency FH.sub.a are set to predetermined values corresponding to the type of the acceleration sensor and/or type of the air bag for use in the automobile. It is also set for the above-mentioned FL.sub.1 similarly.
In order to expand the measuring bandwidth, it is desirable to lower for the above-mentioned low-frequency cut off frequency FL.sub.a (Hz). It also holds the same for the above-mentioned FL.sub.1.
By having miniaturized the acceleration sensor 51, the piezoelectric element is also made smaller. When making the piezoelectric element smaller, the capacitance Cg of the piezoelectric element also becomes smaller. Accordingly, in order to avoid an increase of the low-frequency cut off frequency, as is evident from the equation (1), a resistance value of the leakage resistor Ra has to be set larger.
The amplifier circuit A has a two-stage configuration circuit using the operational amplifiers 52, 53 of the MOS-FET input type. Herein, an output off set voltage of the operational amplifier 52 is reduced by the capacitor Cb, and a signal component in which the output off set voltage is reduced has been amplified in the post stage section of the amplifier circuit having the operational amplifier 53. However, even if adopting the above-mentioned circuit configuration, which uses the operational amplifiers 52, 53 of the MOS-FET input type of which the bias current is small, it is limited to approximately 10 G.OMEGA. for the value of leakage resistor Ra, and the leakage resistor Ra with a larger resistance value than this value could not be used. That is, when using the leakage resistor Ra with more than 10 G.OMEGA., the voltage fluctuation of the operational amplifier 52 becomes larger, thereby making it impossible to secure a large dynamic range.
Further, as for an output signal of the piezoelectric element, it is common to use a charge signal--voltage signal converter that is so-called a charge amplifier, and in that case the capacitance of the piezoelectric element has no influence on the output signal of the piezoelectric element. Thus, it can avoid the above-mentioned problem, but comparing the voltage signal with the charge signal, when using the charge signal, a fluctuation or variation thereof is large, and thus there is a problem such that it is difficult to adjust the charge signal in the circuit.
Moreover, when using the charge signal--voltage signal converter, a number of parts become larger, thereby making the circuit more complicated and bulky.
On the other hand, in the above-described amplifier circuit A, it is hardly affected by a fluctuation of the charge signal, but as described above, it is difficult to implement a further miniaturization of the acceleration sensor. In order to achieve a miniaturization of the acceleration sensor, a high performance such as by using a 2-axis detection system and the like, and a low cost, it is strongly desirable to simplify the configuration of the amplifier. However, in the amplifier circuit A including the above-mentioned two operational amplifiers 52, 53, it is difficult to simplify the circuit, and the number of parts has to be large. Moreover, it requires two expensive operational amplifiers of the MOS-FET input type, which leads to high cost.